Eon ticket
Mix records with a Ruby or Sapphire that has an Eon ticket, and Emerald will get the Eon Ticket. Use it at the SS Tidal to go to Southern Island, after the E4. You'll find a forest area that has an egg in the middle. Press A at the egg to find the Lati@s you did not choose. ex. If you said you saw a Blue Pokemon on the TV(Latios) you will encounter Latias, and vice versa.

Get the Mental Herb
In one of the treehouses in Fortree City is a man with a Wingull. Talk to the Wingull, and it will fly off out of the treehouse. When you get to Mossdeep City, you will see a person with a Wingull. Talk to it and it will fly off as well. Then go back to Fortree City and into the treehouse where you talked to the first Wingull. Talk to the man next to the Wingull, and he will give you the Mental Herb, and item which is a hold item that prevents attraction on the Pokemon that is holding it.

Unlock special pokemon: Mew
Get the Old Sea Chart, and go to Faraway Island

Battle Frontier Catching pokemon
When you first explore the battle frontier, go to the BATTLE PALACE, and go to the southwest corner. SURF to the other side, and you will first see a little old lady. continue down the path south of the lady until you notice a strange tree that moves when you press A. This is a sudowoodo, origianlly found in the GSC versions. To battle it, use the WAILMER PAIL on it. You have only one chance to catch it, so don't screw it up. After that, you can SURF on the water that the Sudowoodo was blocking. SURF down the WATERFALL, and keep surfing left (there are no fish or anything in the water, so don't bother.)

After a while, you will surf to a cave known as the ARTISTAN CAVE. Go inside, and as you explore the cave, you will find many stat items in the boulders (carbos, etc.) The only pokemon in this cave is SMEARGLE. (hence, the ARTISTAN cave.) The SMEARGLE are generally level 41 to 50. They can only use sketch once before they start struggling.

Unlimited Safari Zone Steps
Pay 500 to go to the Safari Zone. Go to a grass patch. Stay in the patch and don't walk. Just press the arrow buttons softly like a 90 degree angle and you might catch a rare pokemon without wasting your 500 steps!

Prizes from Scott
Talk to Scott in his house in the Battle Frontier, he'll give you certain decorations/berries depending on your accomplishments in the Battle Frontier.

Unlock special pokemon: Groudon
Beat the Elite Four, and go to the land lair.

Raising weak pokemon easier
Just put the weak pokemon in the first slot. When a battle starts just switch out that pokemon right away. After the battle the pokemon will recieve half of the exp. points.

Unlock special pokemon: Castform
Take out the rival team at the weather institute.

Special unlockable Pokemon
There are certain pokemon that can only be caught when you trigger certain events, like castform.

Unlock special pokemon: Latios or Latias
After beating the elite four, watch the T.V. downstairs in your house.

Mirage Island
Mirage Island is an invisble Island located on Route 130. Most of the time it will be invisible but on rare cases, the island will be visible. To find out if the island is there or not, go to Pacifidlog Town and talk to the old man in the bottom right house looking out the window. He will tell you wheter the island is there or not. The cause is unknown so a bit of luck is needed to find it. On the island you will find the rare Leichi Berry that powers up your Pokemon's attack in battle when it gets weak. In the grass, you will only find Wild Wynauts.

Unlock special pokemon: Chikorita, Cyndaquil or Tododile
When you complete the 200 Pokemon of Hoenn. Professor Birch will call you on your Pokenav.

National Pokedex
Beat the Elite 4. Proffessor Birch will visit you and give it to you once you leave your house.

Get a Premier Ball free!
Buy exactly 10 Pokeballs at the Poke Mart. You get 1 Premier Ball free.

Unlock National Pokedex Diploma
Capture all 386 pokemon in your pokedex. Then go to the hotel in Lilycove City and talk to the Game Designer.

Altering Cave
You can go to Altering Cave after you beat the Elite Four. It is located on Route 103.

Magnet Pull's secondary effect
If you have a pokemon with the Magnet Pull ability in front of your party, it increases the chances of finding wild steel-type Pokemon. This is very useful for finding rare steel types, such as Mawile.

Unlock special pokemon: Deoxys
Get the Aurara Ticket, and go to Birth Island

Suction cups/Sticky hold's second ability
If a poke'mon in your party has suction cups/sticky hold as an ability, then you will have an easier time fishing for wild poke'mon.

Unlock special pokemon: Latios or Latias
Mix records with a Ruby or Sapphire with the Eon ticket, and go to Southern Island.

Getting Latias Or Latios
After you defeat the Elite Four, go to your house and watch the television downstairs. After you watch it, your mom will ask you what color the Pokemon is. If you say Red, you will find Latias. If you say Blue, it will be Latios.

Synchronize's second ability
If you have a poke'mon with synchronize as an ability, you will encounter more poke'mon with the same nature as the poke'mon with sychronize. Note that the poke'mon with this ability must be in the front of your party.

Masterball: Masterball trick
To use the masterball trick when you throw the pokeball hold down and a when you see the ball open hold a and b (this does not work with all balls)
It works with these:

Pokeball level 20-1
greatball 30-1
Ultra ball 40-1 usually 35-1

Shiny pokemon
Every time you encounter a pokemon you have a 1/8192 chance of the pokemon being a different color, this is called a shiny pokemon

Unlock special pokemon: Ho-oh/Lugia
Get the Mystic Ticket, and go to Navel Island

Mystery Gift
To unlock mystery gift, go to a pokemon mart and press A on the piece of paper on the counter. Then insert the words LINK TOGETHER WITH ALL. Press select to easily find the words. After this the store lady will talk to you and tell you that mystery gift is now unlocked.

Unlock special pokemon: Kyogre
Beat the Elite Four, and go to the sea lair.

Unlock special pokemon: Rayquaza
Get Magma and Aqua to get Kyogre and Groudon out of the cave, and go to Sky Pillar.

Faster hatching eggs
Simply put a pokemon with the ability Flame Body or Magma Armor on your team to cut the amount of steps needed to hatch an egg in half.

Unlock Battle Frontier
Once you beat the game, you can go to the Battle Frontier.

Unlock special pokemon: Beldum
See Steven's house after you beat the Elite four.

Pokerus
This is a helpfull virus in the game. This virus can improve all of your stat points when you grow levels. If your pokemon has pokerus and it's in your party it will spread to your other pokemon. But these effects are temporary unless you put the infected pokemon in your PC. In order to get it, you need A LOT of luck. You need to find a wild pokemon with pokerus. You will know it has pokerus because it will be it's status condition. After you catch it, to make sure it really has pokerus go to any pokemon center and the nurse will say that it is infected with pokerus.

Espeon and Umbreon
In Emerald, there is only ONE way to get Espeon or Umbreon, what you do is first, you trade over an Eevee from FireRed/LeafGreen, then make it love you (Berrys.. Having it not faint, Etc) then, train it one level up during the day (The clock in your room MUST be between 12:00 AM and 11:59 AM) and the Eevee will evolve into Umbreon, train it one level up during night (The clock in your room MUST be between 12:00 PM and 11:59 PM) and the Eevee will evolve into Espeon.

Trainer Card Color Upgrades
As you accomplish more along your journey, your Trainer Card will change colors and rise in rank, depending on completing certain tasks. After completing one task, it will rise in one rank, but there are some tasks that cannot be completed until beating the Elite Four. You must do these tasks:

-Become Pokemon league Champion
-Complete the Hoenn Pokedex
-Collect all the gold symbols in the Battle Frontier
-Win one ribbon in all five of the Master Rank Pokemon Contests.

Bronze Rank - Do one of the tasks mentioned above.
Copper Rank - Do two of the tasks mentioned above.
Silver Rank - Do three of the tasks mentioned above
Gold Rank - Do all four of the tasks mentioned above.

Getting the other Lati
To get the other Lati (Latios or Latias), Mix Records with a Pokemon Ruby/Sapphire game that used the Eon Ticket. You can then get to a hidden island via the SS Tidal where you can find the Lati you didn't choose.

Battle Steven, the former champion.
After beating the Elite Four, a new part of Meteor Falls opens up. It is in the northwestern corner where you found Iron Tail. A new cave was made and you'll find Steven in it. His Pokemon are in the high 70's.

Lightningrod's second ability
If you have a poke'mon with lightningrod as an ability, you will get calls from registered trainers more often.

Pokemon Diploma
Catch all 202 pokemon in your pokedex. Then go to the hotel in Lilycove City and talk to the Game Designer.

Get A Kanto Starter!
After catching 60 Pokemon and beating the Elite 4 at least one time go back to Littleroot Town and talk to Prof. Birch. He will give you the National Dex (your pokedex upgrade), and he will allow you to choose from either Totodile, Chikorita, or Cyndaquil (the three original starters from the Gold, Silver, and Crystal Versions).

Frontier Brains
You can encounter special trainers, such as Pike Queen Lucy, in facilities of the Battle Frontier. Beat them to earn Symbols for your Frontier Pass. The last trainer will be the brain. For example, you beat the Pike once. You're in the 13th room. A woman will tell you the Pike Queen is coming. Go in any room to battle.